We propose to perform x-ray diffraction studies into the physical basis of membrane stability. It is known that phospholipids spontaneously assemble in a variety of bilayer and non-bilayer structures. Our research is aimed at understanding the molecular forces which stabilize bilayers in lieu of non-bilayer structures. This must be understood if the structural roles of different types of biomembrane lipids are to be elucidated. Specifically: 1) The elastic properties of lipid layers will be investigated. Of particular importance is the measurement of a thermodynamic parameter specifying the tendency for a given lipid layer to curl. 2) Correlations between the tendency to curl and destabilization caused by lipophilic agents (e.g., oil, alcohols, anaesthetics and polypeptides) will be examined. 3) The hydration potentials of lipid headgroups will be measured. 4) Synthetic asymmetric lipid bilayers will be made and the phase transition behavior will be studied. 5) The phase transition behavior of novel lipids will be examined.